Stick with a Centrino processors.. They have very low power consumption, and the processor speed is always at max speed, when plugged in or on battery..
My dads laptop is a pentium 4 mobile 1.8 GHZ, when running on battery it reduces its speed to 800 mhz, to increase life, but it runs like shit.

try and get ones with decent ram.. and a decent hard drive..

if you want just for word processing and internet.

check out

www.tigerdirect.ca
they got alot of good deals on older laptops, like p3 700 mhz and the such.. These laptopcs are great for internet, word, spreadsheets, msn, and taking notes all around.

otherwise a descent centrino will running you around 1000$ i think but will have a very good performance/battery life. and if you are going mobile, you always want good battery life.